Extracellular Ca²⁺ is obligatory for ouabain‐induced potentiation of cardiac basal energy expenditure

Abstract

1. The method of action of cardiac glycosides is commonly explained by the 'pump-inhibition hypothesis': inhibition of the Na+/K+-ATPase allows [Na+]i to rise, eventually reversing Na+/Ca2+ exchange. The resulting influx of Ca2+o increases [Ca2+]i, thereby activating intracellular Ca2+-dependent ATPases and, hence, energy demand. This sequence has been presumed to occur during diastole as well as systole. However, it has been reported that dihydro-ouabain-induced potentiation of heat production by quiescent ventricular trabeculae persists in the absence of Ca2+o. This implies that the pump-inhibition hypothesis is inapplicable during diastole. 2. We tested this implication by: (i). measuring the rate of oxygen consumption (Vo2) of arrested guinea-pig whole-hearts; (ii). measuring[Ca2+]i in quiescent ventricular trabeculae; and (iii). mathematical modelling using software (Oxsoft Heart, Oxford Software, Oxford, UK) based on DiFrancesco-Noble formalism. 3. Upon induction of arrest, whole heart Vo2 fell to one-quarter of its 'beating' value. Subsequent perfusion with ouabain (20 micromol/L), in the presence of Ca2+o, increased Vo2 fourfold. This increase was prevented by withholding Ca2+o. Comparable results were obtained in quiescent trabeculae: ouabain increased [Ca2+]i only if Ca2+o was present. Mathematical modelling readily simulated these experimental results. 4. We conclude that influx of Ca2+o is mandatory for potentiation of cardiac basal metabolism by cardiac glycosides.

Systems analysis of digoxin kinetics and inotropic response in the rat heart: effects of calcium and KB-R7943

“…This is consistent with previous results showing that KBR preferentially blocks the Ca 2ϩ influx (reverse) mode of the cardiac NCX rather than the extrusion (forward) mode (2,10,25,39,45) and that KBR inhibits Ca 2ϩ influx through NCX under Na ϩ -loaded conditions in rat myocardium (18,38,41,49). Furthermore, it has been shown that Ca 2ϩ influx (via reversal of NCX) is essential for digitalisinduced potentiation of cardiac basal energy expenditure (15). Note also that the selective inhibition of the reverse mode of NCX is especially pronounced at low concentrations of KBR (2,39,49) and characterized by an IC 50 of 0.15 M in isolated cardiomyocytes (18).…”

“…Representative simultaneous fits of the model to data obtained for three consecutive digoxin doses (15,30, and 45 g) and [Ca 2ϩ ] o of 0.5 and 1.5 mM are depicted in Fig. 5.…”

Ouabain Induces Apoptotic Cell Death Through Caspase- and Mitochondria-dependent Pathways in Human Osteosarcoma U-2 OS Cells

“…Herein, we found that ouabain increased Ca 2+ production, but decreased ROS production and the level of ΔΨ m in U-2 OS cells (Figure 3). Other reports have shown ouabain to induce Ca 2+ production (26) and ROS generation (27) that are involved in glioblastoma cell apoptosis. The contrary results might be due to the different cancer cell types, and further investigations are necessary.…”